Method of producing protective coatings on magnesium and the alloys thereof



Patented Apr. 16, 1940 PATENT OFFICE METHOD OF PRODUCING PROTECTIVECOATINGS ON MAGNESIUM AND THE ALLOYS THEREOF Johannes Fischer, Berlin-Siemensstadt,

and

Werner Richter, Berlin, Germany, assignors to Siemens & Halske,Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a corporation ofGermany No Drawing. Application May 20, 1936, Serial No. 80,908. InGermany June 1, 1935 6 Claims.

It is well known in the art to produce coatings on magnesium with theaid of an electric current which protect this valuable light metalagainst corrosive influences of any kind. This may, for instance, beaccomplished by electrolysis of fused fluoride electrolytes. Thedifficulties and disadvantages presented in the treatment of bathscontaining a fused electrolyte are so Well known that they need not bedealt with here. Of course, attempts have also been made to replace thefused electrolyte by an electrolyte which may be employed at normaltemperature.

Experiments to produce protective coating on magnesium in an aqueoussolution led to the well-known rule of utilizing baths containing alkalihydroxides or alkaline-earth hydroxides.

Experience has shown that this rule cannot be applied .under certaincircumstances. If, for instance, an electrolyte is used which containsonly caustic soda no coating of any practical value can be produced. Itmay even happen that the metallic surface becomes very often pitted.

Accordingly experiments have been carried out to substitute otheralkaline substances for the mordant alkali. In this connection theexperiments with carbonate of alkali as electrolytic component did notgive any satisfactory results. The same applies to the electrolysis incyanide baths.

However, the experiments have surprisingly shown that the electrolytemust contain according to the invention at least three components inorder to attain satisfactory coatings that may be reproduced at will.The three electrolytic components necessary according to the inventionmust be taken from the groups of compounds which present a certainchemical affinity with one another.

To the first group belong besides the alkali metal hydroxides, which arepreferably employed, also the alkaline-earth hydroxides and the aqueoussolution of ammonia. The second group comprises water solublecarbonates, silicates and borates; also the salts of the polyboric acidsmay be employed. Finally the third group in cludes cyanides,thiocyanides and cyanates.

The electrolytes composed according to the invention of at least threesubstances, one from each of the three groups, produce when anodicallytreating magnesium very satisfactory protective layers which, whencoated with 'inorganic or organic sealing means render the metallicsupports highly resistant to corrosion. As sealing means varnish,grease, oil, wax, colloidal hydroxide, silica gel, etc., may beemployed.

The porosity of the novel layers enables the application of a colorwhich may be desirable for certain purposes. Instead of thedye'solutions any other filling material solutions may, of course, beutilized for treating the electrolytically prepared objects ofmagnesium. It should be considered that the porosity of the novel layersdecreases with time. Experiments have shown that the newly producedprotective coatings present a considerably-greater adsorptive power withrespect to coloring substances of any kind than seasoned protectivecoatings. This fact renders it possible to regulate the depth of color.As coloring substances all organic dye solutions may be utilized as wellas colored inorganic substances precipitated in the layer.

The voltage is preferably taken as 16 volts. However, good results mayalso be attained with a gradual change in voltage. Thus a coating wasproduced at a voltage of 16 volts and then the coating was strengthenedat or 110 volts. The strengthening of the coating is possible owing tothe greater residual current resulting from the higher voltages. Finallyalso gradualy increasing voltages may under certain circumstances beemployed; for instance, by gradually increasing the voltage from 1 voltto 60 or 110 volts, While maintaining the intensity of the currentconstant.

The temperature during the treatment is preferably chosen between 20 and25 centigrade, since at a higher temperature a pitting occurs, It is,therefore, necessary to cool the object to be treated during the passageof the current. At temperatures which lie considerably below 20 thecurrent is blocked too quickly so that the formation of a good thickabsorbing layer is prevented. This phenomenon is due to the fact thatthe residual current at low temperatures becomes considerably weaker.The .current density decreases by suddenly during the first seconds ofthe treating period. It is kept at this .low value preferably for 60minutes.

After this treating period the electrolytical process may be consideredas terminated. In particular cases iii which-colored coatings aredesired the objects are then treated with the dye solution and finallythey are sealed; forlnstance,

varnished so as to protect them against corrosion.

While the three above-mentioned groups of substances must be present inthe electrolyte further admixtures from a fourth group may be employedwhen composing the electrolyte which produce then a better coating. Thisfourth group of substances includes the water soluble sulphates,nitrates, phosphates, halogenides, halogenates, fiuosilicates,chromates,-tungstates, etc.

As to the concentration to be used a great advantage is obtained fromthe industrial point of view, since the so-called degreasing baths forelectroplating obtainable on the market present the desiredconcentration for the novel method. Soda, for instance, may be used withsuccess in a 5% solution, pure caustic soda in a 6% solution of NaOH andpotassium cyanide in a 2% solution of KCN.

The cations, panticularly the alkalies, may be replaced by one anotheror uniformly chosen from all three groups; only ammonium forms anexception, since a very bad coating was attained in connection withammonium compounds.

Example 1.-In an aqueous electrolyte containing 5% of NaOH, 5% ofNazCOs, 2% of KCN and 0.1% of Na2WO4 a sheet metal consisting of analloy of magnesium was subjected anodically at a temperature of 20 C. tothe action of the current. At a voltage of 16 volt a current density of5 amp./dm. was attained in the first moment and this decreased to 1amp./dm. within the following 30 seconds. The current treatment lastedan hour. The sheet metal provided with the fresh layer was dipped" intoa dye solution and then coated and varnished. The layer thus producedwas extremely tight and uniformly colored.

Example 2.-Instead of the sodium tungstate employed in Example 1 aselectrolytic admixture, potassium iodide, potassium chromate, sodiumchloride, potassium chlorate, potassium nitrate, sodium silicate andsodium sulphate were alternately used in succession as electrolyticadmixture at a concentration of 0.1%. Particularly good absorbing layerswere obtained when admixing potassium iodide, potassium chlorate andpotassium nitrate. Experiments have shown that an admixture of potassiumiodide and sodium silicate caused a pitting at the surface of -theliquid. Inthe last-mentioned'example a relatively slight porosity of thecoating resulted.

Example 3. The method according to the invention was carried out withoutadmixing a substance of the fourth group. In this case instead of 5% ofNaZCOa, 5% of borax (NazBiov) or 5% of water glass (NazSiOa) was used.The electrolyte consisting of potash lye, soda and potassium cyanidegave without any admixture excellent results which, however, could befurther improved by substituting borax for soda; that is to say-theporosity of the coating is improved. Even without the admixture of asubstance of the fourth group the electrolytes containing water glassproduced a layer of a slighter porosity.

Example 4.-Protective layers were produced in an electrolyte whichcontained 2% of KCNS and 2% of KCNO instead of 2% of KCN. In the firstcase a very hard coating which was substantially free of pores wasproduced, whereas when using 'KCNO as electrolytic component a goodaverage result was obtained. I

Example 5.Finally the same cation was employed forall components. Anexperiment was carried out with 5% of KOH, 5% of K2CO3, 2% of KCN ad'0.1% of K2804. The absorptive power of he layer thus produced wasabove average.

The expression the simple cyanides, thiocyanides and cyanates of thealkali metals as used in the following claims, is intended to excludecompounds such as the so-called complex, compound or double cyanides ofthe alkali metal cyanides and cyanides of the heavy metals.

We claim as our invention:

1. The method of producing non-metallic protective coatings onmagnesium, which comprises anodically treating the magnesium in anaqueous solution of a mixture consisting substantially of hydroxideselected from the group consisting of alkali metal hydroxides, alkalineearth hydroxides and aqueous ammonia, and salt selected from the groupconsisting of water soluble carbonates, silicates and borates, and saltselected from the group consisting of the simple cyanides, thiocyanidesand cyanates of the alkali metals.

- 3. The method of producing non-metallic protective coatings onmagnesium, which comprises anodically treating the magnesium at between1 and volts at a temperature of about 20 to 25 C. in an aqueous solutionof a mixture consisting substantially of hydroxide selected from thegroup consisting of alkali metal hydroxides, al-

- kaline earth hydroxides and aqueous ammonia,

and salt selected from the group consisting of water soluble carbonates,silicates and borates, and salt'selected from the group consisting ofthe simple cyanides, thiocyanides and cyanates of the alkali metals.

4. The method of producing non-metallic protective coatings onmagnesium, which comprises anodically treating the'magnesium at about 16volts in an aqueous solution of a mixture consisting substantially ofhydroxide selected from the group consisting of alkali metal hydroxides,alkaline earth hydroxides and aqueous ammonia, and salt selected fromthe group consisting of water soluble carbonates, silicates and borates,

' and salt selected from the group consisting of the simple cyanides,thiocyanides and cyanates of the alkali metals. 4

, 5. The method of producing non-metallic protectivecoatings onmagnesium, which comprises anodically treating the magnesium at avoltage increasing from less than 16 volts to at least about 60 volts inan aqueous solution of a mixture consisting substantially of hydroxideselected from the group consisting of alkali metal hydroxides, alkalineearth hydroxides and aqueous ammonia, and salt selected from the groupconsisting of water soluble carbonates, silicates and borates, and saltselected from the group consisting of the simple cyanides, thiocyanidesand cyanates of the alkali metals.

6. The method of producing non-metallic pro- 15 aroma a 3 tectivecoatings on magnesium, which comprises anodically treating the magnesiumin an aqueous solution of a mixture consisting substantially ofhydroxide selected from the group consisting of I alkali metalhydroxides, alkaline earth hydroxides and aqueous ammonia, salt selectedfrom the group consisting of water soluble carbonates,

silicates and borates, salt selected from the group consisting ofthesimple cyanides, thiocyanides highly dissociated acid, theconcentration of said 5 latter ingredient being lower than that of saidthree groups.

JOHIANNES FISCHER. WERNER. RICH'IER.

